Color tunable OLED devices frequently comprise stacked OLED diodes having different emission colors. Such stacked OLEDs contain so-called charge generation layers as transparent electrodes separating the different diodes which are connected to the outside. By applying a voltage to the individual OLED diodes of the stack, the color can be tuned over a wide range.
One disadvantage of such color tuneable stacked OLED devices is that the electrodes have to be highly transparent and should therefore have a low sheet resistance. However, a high transparency and a low sheet resistance are contradictory requirements. Therefore, in most practical cases a compromise between sheet resistance and transparency has to be found which makes large area color tuneable OLEDs difficult to realize. The whole OLED stack is also rather complicated since stacked OLEDs contain typically a large number of layers.
Another technique to realize color tuneable OLEDs is to individually address OLED pixels of different emission color. This is a common technique in displays, which is of course expensive for a light source. If the pixels are made of large size an additional scattering screen is required to get uniform light of one specific color.